P.
153 A careful survey of the character of the rocks in the keys
affords satisfactory evidence that they have been formed at
whatever height they may rise , by the same action which is
now going on upon the reef--that is, by the accumulation of
loose materials above the water-level. That part of the keys
which rises above the level of the water is, therefore, a sub-aerial
and not a submarine accumulation of floating matter, thrown
above high-water mark by the tempestuous action of the water.
We insist upon the fact, that the keys furnish in themselves,
by the internal structure of their rock, the fullest evidence
that they have been formed above high-water mark by the action
of gales and hurricanes, instead of having grown as a reef up
to the water-level, and been subsequently raised to their present
height. The evidence of this statement rests upon certain facts
obtained from observations of the reef itself, at Sand key and
the Sambos.

Coral
Reefs

P.
153-154 After examining a growing coral reef, so full of life,
so fresh in appearance, so free from heterogeneous materials,
in which the corals adhere so firmly to the ground, or if they
rise near the surface seem to defy the violence of the ocean,
standing uninjured amid the heaviest breakers, an observer cannot
but wonder why, in the next reef, the summit of which begins
to rise above the level of the water, the scene is so completely
changed. Huge fragments of corals, large stems, broken at their
base, gigantic boulders, like hemispheres of Porites, and Macandrina,
lie scattered about in the greatest confusion--flung pell-mell
among the fragments of more delicate forms, and heaped upon
those vigorous madrepores which reach the surface of the sea.

The
question at once arises, how is it that even the stoutest corals,
resting with broad base upon the ground, and doubly secure from
their spreading proportions, become so easily a prey to the
action of the same sea which they met shortly before with such
effectual resistance? The solution of this enigma is to be found
in the mode of growth of the corals themselves. Living in communities,
death begins first at the base or centre of the group, while
the surface or tips still continue to grow, so that it resembles
a dying centennial tree, rotten at the heart, but still apparently
green and flourishing without, till the first heavy gale of
wind snaps the hollow trunk, and betrays its decay. Again, innumerable
boring animals establish themselves in the lifeless stem, piercing
holes in all directions into its interior, like so many augers,
dissolving its solid connexion with the ground, and even penetrating
far into the living portion of these compact communities. The
number of these boring animals is quite incredible, and they
belong to different families of the animal kingdom: among the
most active and powerful we would mention the datefish, Lithodomus,
several Saxivaca, Petricola, Arca, and many worms, of which
Serpula is the largest and most destructive, inasmuch as it
extends constantly through the living part of the coral stems,
especially in Macandrina.

On
the loose basis of a Macandrina, measuring less than two feet
in diameter, we have counted not less than fifty holes of the
date-fish--some large enough to admit a finger--besides hundreds
of small holes made by worms.

But
however efficient these boring animals may be in preparing the
coral stems for decay, there is yet another agent, perhaps still
more destructive . We allude to the minute boring-sponges, which
penetrate them in all directions, until they appear at last
completely rotten throughout.

Coast Survey

But
it may be asked, what is the practical use of such detailed
descriptions of the coral reefs for the coast survey? We need
only allude to the universal impression of the dangers arising
to navigation, from the growth of such reefs, to satisfy the
most skeptical that a minute knowledge of the extent and mode
of formation of those belonging to our own shores must be of
paramount importance, were it only with reference to the position
of light-houses.

Changes
in ages to come

Among
the questions contained in your instructions, you ask whether
the growth of coral reefs can be prevented, or the results remedied,
which are so unfavorable to the safety of navigation. I may
say that here, as in most cases where the operations of nature
interfere with the designs of man, it is not by a direct intervention
on our part that we may remedy the difficulties, but rather
by a precise knowledge of their causes, which may enable us,
if not to check, at least to avoid the evil consequences. I
do not see the possibility of limiting in any way the extraordinary
increase of corals, beyond the bounds which nature itself has
assigned to their growth.

......we
may therefore rest assured that the changes which are going
on will chiefly consist in bringing up the reef, for its whole
extent, to the surface of the water, with occasional intervening
channels kept open by the currents, such as exist now between
the keys; that this reef once matured, will be covered by coral
debris, becoming transformed into a range of keys, similar to
that which exists now inside of it; that the depth of the ship
channel between the reef and main range of keys will gradually
lessen, and the channel itself be changed into mud flats, similar
to those stretching now between the keys and the main land.
In still more remote ages the present mud flats may become swamps,
elevated above the reach of the tide-waters, like the everglades;
and this process may perhaps be extended to the present ship
channel. But unless some great revolution in nature modifies
the present relative level between land and sea, it may safely
be maintained that the present outer reef is the final southern
boundary of the North American continent, and that the sooner
a system of light-houses and signals is established along the
whole reef, the better; for this is, after all, the shore which
is to be lighted, and not the range of keys which is within
the reef.

These
practical results--for so we venture to call the general conclusions
last presented-- although they are purely scientific deductions
from general principles, may satisfy the most obstinate supporters
of the matter-of-fact side of all questions, of the advantages
of scientific illustrations in the daily walks of life, and
also justify the course which has been followed with so much
success by the Coast Survey, in combining the strictest scientific
methods with its practical operation.